Oil and gas related projects often include large pump and/or compressor drives that are designed to be suitable for cost efficient construction, start-up and operation under various conditions. A relevant project example would be a LNG-plant.
Liquefied Natural Gas or LNG is a product from the cooling down of natural gas to a temperature at which the natural gas becomes liquid. Natural gas can be transported in large volumes in liquefied state in an economic manner, particularly in situations where pipelines are not available or are to expensive or time consuming to install. Typically, LNG tankers transport large volumes of natural gas by sea from a production site to a consumption site or an intermediate storage site.
International Patent Application Publication no. WO 97/33131 describes an installation for producing liquefied natural gas, where the main cooling stages are mechanically interconnected and are arranged to be driven by a single common gas turbine, with an auxiliary engine being arranged for start-up of the gas turbine. Disadvantages of mechanically interconnecting the driving and driven parts of the installation are, among others a long train of rotating machines on a common shaft, which takes considerable space and requires a carefully balanced shaft, and that the driving and driven machines must be situated close to each other.
At present the production of LNG is often based on gas and or steam turbines to provide power to drive the refrigeration processors used to liquefy the natural gas. As an example, International Patent Application Publication no. WO 2005/047789 A2 describes a drive system for LNG production in which a refrigeration compressor is driven by a gas turbine coupled together with an electric starter motor on a common drive shaft running between gas turbine and the compressor.
Compressor drives based on a gas turbine as a mechanical driver located relatively near to the compressor, have the hazards related to a gas burning device normally operating at a fairly high temperature arranged fairly close to a refrigeration compressor in an LNG-plant. Further, the gas turbine normally produces an amount of exhaust gas which as to be dealt with in some manner to limit the hazards in a natural gas processing plant. Anyone that considers constructing or operating LNG-plants onshore, offshore or anywhere else will understand that local gas burners and hot exhaust gas emissions in relation to hazardous areas is not an ideal solution, and a replacement solution that also could improve production regularity would most likely be attractive. The obvious alternative option is frequency controlled electric drives that also has to be used in various combinations with turbine drives. The plant operated by the company Statoil at Hammerfest in Norway is an example of a frequency controlled LNG-plant.
U.S. Pat. No. 5,689,141 describes a compressor drive system for a natural gas liquefaction plant having a frequency converter assisted start by acceleration of the gas turbine to ram-speed and ignition. During normal operation at fixed speed the electric motor works as a generator to convert excess mechanical power generated by a gas turbine into electrical energy and feeding this electric energy to the main power source of the plant. In this case the gas turbine and compressor are assembled on a common shaft.
A frequency converter is a quite large piece of equipment. To transfer power rates in the order of 50 MW the frequency converter could fill a four or five storey building with a base of about 1000 m2. Despite the size and costs of these frequency converters, they have often been considered necessary equipment when transferring power electrically between driving and driven equipment when the power rate is in the order of 10 MW or more.
U.S. Pat. No. 6,640,586 B1 describes a natural gas liquefaction process system employing mechanical drivers and electric motors as compressor drivers. Various combinations of compressors and electrical motors are described, used for various types of refrigerants. This publication includes a brief description of several drawbacks of using conventional gas turbines in LNG plants and discloses various ways of combining compressors and electric motors for driving the compressors in an LNG plant. The electric motors are in these solutions often used for startup by various methods and possibly to assist gas turbines during normal operation and the mechanical driven string some times also include steam turbines.
In view of the above described prior art and the drawbacks pointed out in the closest prior art it is desirable to provide new and inventive solutions which can overcome one or more of the above cited drawbacks, while obtaining optimum functionality in a compressor driver system for an LNG plant or other purposes.